Description and Vorticity Analysis of 50-Day Oscillations in the Western Tropical Region of the CME Model

Abstract

The WOCE Community Modeling Effort (CME) general circulation model of the North Atlantic was used to investigate the behavior, nature, and dynamics of 50-day oscillations seen in the meridional component of velocity between 5° and 11°N, 35° and 55°W. Oscillations of the meridional component of velocity, with a period of about 50 days, appear as the seasonal meander pattern of the North Equatorial Countercurrent starts to break down in December. They appear first near 35° and are advected westward. They have a westward phase velocity of about O.1 m s?1, wavelength of about 6000 km, and a very slow eastward group velocity. Their period, phase speed and wavelength agree with recent observations. Calculation of the leading terms from the full vorticity equation following a model decomposition in the propagation region showed that the oscillations were first and second mode baroclinic Rossby waves. Repetition of the vorticity analysis on an undecomposed snapshot during the period of retroflection revealed the NECC meanders also to be baroclinic Rossby waves, the same as the 50-day oscillations. These findings, together with the time evolution of the individual flow components over an annual cycle, suggested that the 50-day oscillations were the westward advected residue of the NECC meander pattern that is released as the NECC slows in December. The retroflecting North Brazil Current produces Rossby waves with very slow eastward group velocity that are advected eastward by the NECC until they reach 35°W, where they dissipate. A standing wave pattern is established for several months, while the NECC is active. Once it slows, the waves am advected westward and disappear totally by May. Neither wind forcing nor barotropic instability were considered to be responsible for the oscillations in the mode1.